Since the 1990s, membrane switches have been widely used in the industrial market, especially in household appliances such as washing machines, due to their good appearance, light weight, small size, and long lifespan. The process of using pressure-sensitive adhesive to assemble the keyboard film switch (hereinafter referred to as the disc film) onto the washing machine housing is simple, convenient, and has suitable connection strength, which is currently one of the mainstream assembly methods. With the increasing improvement of people's living standards, the annual production scale of washing machines in China has reached more than 70 million units. The occasional problem of disc film warping not only affects the satisfaction of individual consumers, but also may seriously affect the brand image of home appliance manufacturers. This is a problem that major home appliance brands attach great importance to and are eager to solve.

Analysis of the causes of membrane warping

The membrane switch of the washing machine is a combination of a mechanical micro switch with discrete components and a membrane panel. During operation, the function is achieved by driving the buttons below the panel through a thin film panel with high elasticity. The structure of the entire bonding system includes a disc film, pressure-sensitive adhesive, and a casing.

The disc film is generally made of flexible and transparent polyester (PET) film, with button patterns printed on the back. The casing is injection molded from molds, commonly made of materials such as engineering plastics (ABS) or polypropylene (PP), and the surface of the panel area is relatively flat. Pressure sensitive adhesive often uses non substrate film or PET substrate double-sided tape, which connects the disc film and the chassis together through adhesive force, and opens holes at the button for button function.

When there is external or internal stress present and this force is greater than the actual adhesive strength of the pressure-sensitive adhesive (hereinafter referred to as the tape), the tape will first separate from the side with lower adhesive strength, resulting in adhesive failure.

Improvement approach for disc film warping - plasma surface treatment

All material surfaces have a certain surface energy. The adhesive materials with different surface energies have a significant impact on the degree of tape infiltration. When the surface energy of the material is higher than that of the tape, the material is easily adhered by the tape and a high bonding strength is easily established. When the surface energy of the material is lower than that of the tape, the material is not easily adhered by the tape and it is difficult to establish adhesive strength. Plasma surface treatment of plastic parts is also a very effective measure to improve adhesive strength.

The cleanliness of the material surface directly affects the bonding effect of the tape. The presence of foreign objects such as surface dust, dirt, grease, and moisture can hinder the contact between the adhesive tape and the bonding surface, thereby affecting the establishment of strength. Especially plastic products are prone to residual release agents on the surface during the molding process. The presence of release agent can significantly affect the wetting ability of the tape and affect the bonding strength.

Plasma surface treatment is achieved by generating high-voltage and high-frequency energy discharge through a plasma generator, producing low-temperature plasma, and spraying the plasma onto the surface of the workpiece with compressed air, resulting in physical changes and chemical reactions on the surface of the workpiece. Therefore, the surface of the plastic parts has been cleaned to remove hydrocarbon pollutants such as grease, auxiliary additives, etc. At the same time, certain polar groups (hydroxyl, carboxyl) can be introduced, which are beneficial for the adhesion of the tape. Again, plasma surface treatment can significantly improve the surface energy of plastic parts. After plasma surface treatment, the surface energy of ordinary plastics can reach over 60 dynes, which belongs to the high surface energy range and is very conducive to adhesive tape bonding.

Plasma surface treatment machine activates plastic substrates at room temperature, avoiding the problem of deformation of high-temperature plastic substrates. After low-temperature plasma surface treatment, the adhesion performance of these plastics can be significantly improved. The application of plasma treatment technology has strong practical value and guiding significance for improving adhesion failure problems such as disc film warping.